The invention relates to a drive device which is provided, in particular, for driving a movable part of the closing unit or of the injection unit of a plastic injection molding machine.
Within the closing unit of a plastic injection molding machine, the drive device moves the movable mold clamping plate of the machine. Such a drive device must fulfill two important different requirements. On the one hand, it is to move the mold clamping plate as quickly as possible in order to close and to open the mold, so that the cycle time for producing a molding can be kept short. On the other hand, it is to be capable of keeping the mold clamping plate and consequently the entire mold shut with great force against the high injection pressure. On the one hand, therefore, adjusting movements are to be executed at high speed and, on the other hand, high forces are to be exerted without any appreciable movement. Requirements of this kind may arise not only with regard to the closing unit, but also in respect of the injection unit of a plastic injection molding machine. For example, when plastic is being injected into the mold, the plasticizing worm is moved at relatively high speed in the direction of the mold, until the mold is filled completely with plastic. When the plastic melt contained in the mold is subsequently exposed to what may be referred to as dwell pressure, the drive has to apply a high force without any appreciable movement of the plasticizing worm.
U.S. Pat. No. 4,030,299 discloses a purely hydraulic drive for the movable mold clamping plate of a plastic injection molding machine, said drive also containing a hydraulic force intensifier. The latter has a movable piston with a small active surface, a further movable piston with a large active surface and a cylinder which, together with the pistons, encloses a pressure space filled with a pressure fluid. The cylinder is arranged at a fixed location on the stand of the injection molding machine. The drive includes, moreover, hydraulic cylinders which move the movable mold clamping plate in order to close and open the mold. When the mold is in the open state, the volume of the pressure space of the hydraulic force intensifier is minimal. If, then, the movable mold clamping plate is moved by the hydraulic cylinders with the effect of a closing of the mold, the large piston of the hydraulic force intensifier is taken up, the volume of the pressure space of the hydraulic force intensifier increasing and pressure medium flowing out of a reservoir into the pressure space via a suction follow-up valve. Thereafter, the small piston of the hydraulic force intensifier is moved into the pressure space, thereby generating a high pressure which brings about a high closing force via the large active surface of the large piston. The small piston is moved hydraulically due to the supply of pressure fluid. Thus, in the drive device according to U.S. Pat. No. 4,030,299, various hydraulic drive components are present for the adjusting movement of the movable mold clamping plate and for exerting a high force. During the adjusting movements of the mold clamping plate, a large amount of pressure fluid flows back and forth between the pressure space and the reservoir, thus necessitating correspondingly large valves and fluid ducts.
A drive device is known from DE 41 11 594 A1. In this drive device, a hydraulic cylinder having a large active surface is firmly connected to the movable mold clamping plate. The unit consisting of the movable mold clamping plate and of the hydraulic cylinder can be moved by an electric motor, via a mechanism comprising a lifting spindle and a spindle nut, in order to close the mold quickly and open it quickly. The high closing force is applied by the action of pressure upon the hydraulic cylinder movable together with the mold clamping plate. In this case, the entire reaction force is diverted to the machine stand via the spindle and the spindle nut. The spindle must therefore have a highly robust design, is overdimensioned for the adjusting movement and is correspondingly costly.